1. Field of the Invention
This invention relates to a process for the manufacture of pneumatic radial tires in which an annular groove having a wide width and a deep depth is arranged in an approximately central portion of a tread portion in widthwise direction thereof and continuously extends in a circumferential direction of the tire. More particularly it relates to a process for the manufacture of a pneumatic radial tire capable of developing an excellent drainage property even at a completely worn state of a tread portion in a tire product.
2. Description of the Related Art
As this type of the tire, there have hitherto been proposed pneumatic radial tires as disclosed, for example, in JP-A-4-243601, JP-A-4-243602, JP-A-4-243603 and the like. These tires can develop an excellent drainage property even at the completely worn state of the tire tread.
In these conventional techniques, however, it is necessary that a constricted portion is disposed in an approximately central portion of each of carcass and belt in the widthwise direction thereof for the formation of a large annular groove located at the central portion of the tread.
For this end, it is proposed to form such a constricted portion in each of the carcass and belt in the vulcanization building of an ordinary green tire. In this case, the given constricted portions are formed in the carcass and belt during the formation of the large annular groove through a vulcanization mold when the carcass and belt somewhat protrude outward in the radial direction of the green tire at a transverse section thereof and are vulcanized in the same manner as in the conventionally ordinary radial tire. Thus, the depth of the annular groove can be deepened in accordance with the constricted portion formed in the carcass and belt. Consequently a groove volume enough to provide a given drainage property can still be ensured owing to the presence of the constricted portion even at the complete worn state of the tread.
However, when the above forming is applied to the carcass, belt and the like in the vulcanization building, it is very difficult to ensure a sufficient constricted amount, which undesirably affects size expansion ratio of each portion in a tire product, tension of each belt layer and the like after the vulcanization building.
That is, in the above vulcanized tire, the size expansion amount in the vulcanization building is small in the constricted portion of the belt, while the size expansion amount becomes large in the portion of the belt located apart from the constricted portion, so that the constricted portion of the belt is largely expanded as compared with the other portion of the belt when an air internal pressure is applied to the tire product. Consequently the pushing-out of the groove bottom in the annular groove or the decrease of the groove depth in the annular groove is not avoided and it is difficult to ensure the sufficient groove volume at the complete worn state of the tread.
On the other hand, it is considered to control the decrease of the annular groove depth in the inflation of the tire under an internal pressure by pressing down the outer periphery of the constricted portion formed in the belt with a special reinforcing layer comprised of cords substantially extending in the circumferential direction of the tread. In this case, it is obliged to reduce the initial depth of the annular groove only by the thickness of the reinforcing layer, which is also difficult to ensure the sufficient groove volume at the complete worn state of the tread.